The present invention relates to a storage battery in which an expand grid is used as a battery plate.
A grid which is to be used as a battery plate for a lead storage battery is sometimes produced by the expansion process. Methods of producing an expand grid by the expansion process are roughly classified into two kinds or the rotary method and the reciprocal method.
In the reciprocal method, an expand grid is produced in the following manner. Dice cutters which are arranged in a stepwise manner are vertically moved on a metal sheet that is intermittently moved, to sequentially form slits in the metal sheet, and the metal sheet is then stretched to be formed into a net-like shape. Specifically, as shown in FIG. 11, a metal sheet 1 made of lead or a lead alloy is intermittently transported on a flat upper face of a lower table 2 in the direction of the arrow F. Step-like side faces 2a are formed on the side faces of the lower table 2, respectively. In the step-like side faces 2a, a large number of steps (in the figure, only four steps are shown for the sake of simplicity) are formed in such a manner that the distance between the side faces is stepwise reduced toward the center by a constant step difference as advancing in the direction of the arrow F. An upper table 4 to which dice cutters 3 are attached is placed above the lower table 2. In practice, the upper table 4 is placed in a position which is lower in level than the illustrated position, or which is slightly above the metal sheet 1 transported on the lower table 2, and conducts vertical motions in the position. Step-like side faces 4a which are similar to the step-like side faces 2a of the lower table 2 are formed on the side faces of the upper table 4, respectively. The dice cutters 3 are fixed to the step-like side faces 4a of the upper table 4, respectively, with the result that the dice cutters are arranged in a substantially V-like shape as a whole. In each of the dice cutters 3, an edge 3a is downward projected from the lower face of the upper table 4.
Each time when the intermittent motion is stopped, the upper table 4 is lowered to conduct one cycle of the vertical motion, whereby the end portions of the metal sheet 1 are cut and downward stretched by the edges 3a of the dice cutters 3, resulting in that an expand grid such as shown in FIG. 12 is formed. Namely, the metal sheet 1 is processed into an expand grid in which the both side portions of a collector frame portion 1a in a center area in the width direction are sequentially stretched into grid wires 1b that are connected to one another in a net-like shape. The expand grid has a large number of meshes 1c in the form of meshes each surrounded by four grid wires 1b. The collector frame portion 1a is an area of the metal sheet 1 in which the meshes 1c are not formed in order to enable current collection in a battery plate, and a plate lug for future connection to a terminal is formed. The expand grid shown in FIG. 12 is produced by an apparatus in which, unlike that shown in FIG. 11, twelve dice cutters 3 are attached to each of the side faces of the upper table 4.
In the reciprocal production method, the operations of cutting the metal sheet 1 by the dice cutters 3, and stretching and expanding the grid wires 1b to form the meshes 1c are completed by one cycle of vertical motions of the upper table 4. Therefore, the reciprocal production method is conducted so that each of the meshes 1c is formed into a rhombic shape and the four grid wires 1b surrounding the mesh have the same length, thereby allowing stress in the process of stretching the grid wires 1b to be uniformly applied to the wires. As in the invention disclosed in Japanese Patent Publication (Kokai) No. SHO57-90873, another reciprocal production method is known in which each of the meshes 1c is formed into a substantial parallelogram shape so as to have long and short edges of different lengths. In the reciprocal production method, the grid wires 1b are stretched straight downward by the edges 3a of the dice cutters 3, and hence the grid wires 1b are not twisted during the expansion process. Therefore, the method has the advantage that the grid used as a battery plate of a storage battery exhibits an excellent life performance.
In the expand grid, the meshes 1c on an oblique line indicated by, for example, the one-dot chain line L1 are formed at one stroke by the dice cutters 3 which are arranged in a substantially V-like shape. When the metal sheet 1 is transported by a predetermined distance as a result of intermittent motion and the upper table 4 conducts the next vertical motions, the meshes 1c on the oblique line indicated by the one-dot chain line L2 are formed at one stroke. In the metal sheet 1, therefore, the meshes 1c in the lateral end portions in the width direction are first formed, and, each time when the intermittent motion is further advanced, inner meshes 1c are sequentially formed. The edges 3a of the dice cutters 3 press down two grid wires 1b which are arranged in a substantially V-like shape below the respective meshes 1c. The grid wires 1b which are pressed down by the same dice cutter 3 are arranged in one row along the advancing direction F while being alternately inclined in a zigzag manner.
In the thus produced expand grid, as shown in FIG. 12, the grid wires 1b are connected to one another in a net-like shape on both the sides of the collector frame portion 1a which is formed in the center area in the width direction of the metal sheet 1. When the expand grid is to be used as a battery plate, the collector frame portion 1a is divided into two portions along a cutting line which elongates in the direction of the arrow F. In the resulting expand grid which will be used as a battery plate, therefore, the grid wires 1b in a net-like shape are connected to one side of the collector frame portion 1a. In the reciprocal method of producing an expand grid, since the metal sheet 1 is intermittently transported, the rate of production is somewhat low.
In the rotary method, an expand grid is produced in the following manner. In a slit forming step, first, a large number of zigzag slits are formed in a metal sheet by using a disk cutter. In an expanding step, then, the metal sheet is expanded in the width direction to stretch the slits into a net-like shape. Namely, in the rotary production method, an expand grid is produced in the following manner. In the slit forming step shown in FIG. 13, first, the metal sheet 1 is passed between upper and lower disk cutter rolls 6 each of which is configured by a stack of a large number of disk cutters 5, thereby forming slits 1d. As shown in FIG. 14, each of the disk cutters 5 is a metal disk in which many ridges 5a and valleys 5b are alternately formed in the peripheral face. In peripheral edge portions of the front and rear faces of the disk cutter 5, grooves 5c are formed respectively for the valleys 5b so as to be opened in the corresponding valleys 5b. In each of the valleys 5b, however, the groove 5c is formed in only one of the front and rear faces, and, in the adjacent valleys 5b, the grooves 5c are formed in opposite ones of the front and rear faces, respectively. As shown in FIG. 15, each of the disk cutter rolls 6 is configured by stacking a large number of such disk cutters 5 on the same shaft via spacers 7. The upper and lower disk cutter rolls 6 are placed in positions where the disk cutters 5 are shifted in the axial direction by a half pitch, and the upper and lower peripheral edges are alternately engaged with each other. The upper and lower disk cutter rolls 6 are rotated in synchronization, in opposite directions, and in a phase relationship in which the ridges 5a and the valleys 5b of the upper and lower disk cutters 5 are overlapped and engaged with each other.
When the metal sheet 1 is passed between the disk cutter rolls 6, as shown in FIG. 13, a large number of slits 1d are formed by the disk cutters 5. In the valleys 5b of the upper and lower disk cutters 5 where the grooves 5c face each other, the slits 1d are intermitted, and hence are not continuous along the advancing direction F of the metal sheet 1 but formed with being intermitted at regular intervals. Moreover, the slits 1d which are formed adjacently in the width direction of the metal sheet 1 are shifted by a half pitch in the advancing direction F, and the slits are formed in a zigzag manner as a whole. The thin metal wire-like portions between the adjacent slits 1d are formed as grid wires 1b, and the intermittent portions of the slits 1d along the advancing direction F are formed as nodes 1e. 
Since the grid wires 1b are pressed in the upward and downward directions when the slits 1d are formed by the ridges 5a of the upper and lower disk cutters 5, the grid wires 1b are elastically deformed so as to protrude in the upward and downward directions from the front and rear faces of the metal sheet 1 as shown in FIG. 16(a). All of a series of the grid wires 1b which are arranged via the nodes 1e along the advancing direction F are upward pressed by the ridges 5a of, for example, the lower disk cutter 5 as shown in FIG. 16(b), whereby the center areas are formed as upward protrusions P. All of the series of the grid wires 1b which are adjacent to the grid wires in the advancing direction F of the metal sheet 1 are downward pressed by the ridges 5a of the upper disk cutter 5, whereby the center areas are formed as downward protrusions P.
In the above-described rotary production method, the slits 1d are formed by passing the metal sheet 1 between the two upper and lower disk cutter rolls 6 which are vertically arranged. Alternatively, the slits 1d may be formed by passing the metal sheet 1 between three or more disk cutter rolls 6.
The metal sheet 1 in which the slits 1d are formed as described above is stretched to both the sides in the width direction to be expanded in an expanding step shown in FIG. 17, to be formed into an expand grid. In a usual expand grid which is produced by the rotary method, as shown in FIG. 18, the collector frame portion 1a is disposed in the center area in the width direction of the metal sheet 1, lower frame portions 1f are disposed in the lateral end portions, respectively, and the large number of meshes 1c in a net-like shape are formed between the collector frame portion 1a and the lower frame portions. In the collector frame portion 1a and the lower frame portions 1f, the meshes 1c of the metal sheet 1 are not formed. A plate lug which will be connected to a terminal for the purpose of current collection is formed on the collector frame portion 1a. Each of the lower frame portions 1f is a portion which will function as the lower end of a battery plate when the plate is placed in a battery case. As shown in FIG. 17, the metal sheet 1 is expanded by further laterally pulling the lower frame portions 1f in the lateral end portions by expanding apparatuses 8. The expanding apparatuses 8 are endless chain apparatuses which are placed so as to form a fan-like shape on respective sides of the transportation path of the metal sheet 1. Engagement portions attached to chain rollers are engaged with the lower frame portions 1f of the transported metal sheet 1, so that the metal sheet is stretched obliquely outward. Therefore, the metal sheet 1 is pulled toward the lateral end portions in the width direction, so that the gaps between the slits 1d are widened to be formed into the meshes 1c of a substantially rhombic shape and the four grid wires 1b which surround each of the meshes 1c and which have a substantially same length are connected to one another in a net-like shape, thereby producing an expand grid. The grid wires 1b which are formed by the series of adjacent slits 1d are in the same row, and arranged in a row along the advancing direction F while being alternately inclined in a zigzag manner.
When the thus produced expand grid is to be used as a battery plate, the collector frame portion 1a which is in the center area in the width direction is divided into two portions along a cutting line which elongates in the direction of the arrow F. In the resulting expand grid which will be used as a battery plate, therefore, the grid wires 1b in a net-like shape are connected to one lateral side of the collector frame portion 1a, and the lower frame portion 1f is in the lateral end portion of the grid.
In the rotary production method, the slit formation and the expansion are conducted while the metal sheet 1 is continuously transported. Therefore, the method has an advantage that the speed of producing an expand grid can be made larger than that in the reciprocal method. Unlike the case of the reciprocal method in which the cutting and expansion of the grid wires 1b are completed at one stroke, however, the grid wires 1b of the metal sheet 1 suffer two times high stress due to the slit forming step and the expanding step, because, in the slit forming step, the grid wires 1b are deformed in either of the upward and downward directions by the ridges 5a of the disk cutters 5, and, in the expanding step, are stretched in order to form the meshes 1c. In the expanding step, moreover, unlike the case of the reciprocal method in which the grid wires 1b are pressed only in the downward direction by the dice cutters 3, the grid wires 1b are stretched via the nodes 1e while being twisted. Therefore, also stress due to the twisting is applied to the expand grid. Consequently, an expand grid produced by the rotary method has a further disadvantage that the grid wires 1b are easily ruptured or cracked during the production process to lower the production yield and impair the life performance.
In both expand grids which are produced respectively by the reciprocal method and the rotary method, usually, the grid wires 1b have a uniform width in every portion. In the case where such an expand grid is used as a positive plate of a lead storage battery, however, an oxidation reaction of lead or a lead alloy occurs during a charging process, and the reaction proceeds to sometimes cause the grid wires 1b to be cracked by oxidation corrosion. When the grid wires 1b are cracked, an active material held in meshes 1c which are remoter from the collector frame portion 1a than the cracked portion is electrically isolated to be hardly charged and discharged. When the grid wires 1b in the vicinity of the collector frame portion 1a are cracked, therefore, a larger quantity of the active material are electrically isolated, with the result that the capacity of the battery plate is greatly reduced. To comply with this, conventionally, a countermeasure is sometimes taken in which the grid wires 1b of rows that are closer to the collector frame portion 1a are made larger in width. As described above, the influence due to a crack of corrosion is larger as the cracked row is closer to the collector frame portion. Namely, techniques such as described below have been proposed. In the invention disclosed in Japanese Utility Model Publication (Kokai) No. SHO61-66864, thicker grid wires are formed in a part of an expand grid. In the invention disclosed in Japanese Patent Publication (Kokai) No. HEI1-204364, the widths of grid wires of an expand grid are gradually reduced as proceeding from the upper side to the lower side. In the invention disclosed in Japanese Patent Publication (Kokai) No. HEI9-223502, the thickness of grid wires is defined.
In a positive plate of a lead storage battery for a communication field or the like in which a long life period is particularly requested, an expand grid is sometimes used in which the metal sheet 1 of a large thickness is used so as to thicken all the grid wires 1b, whereby the corrosion resistance is enhanced in order to prevent the grid wires 1b from being cracked by corrosion.